A conventional frequency detector compares the frequency of an input clock with the frequency of a reference clock to determine whether the input clock is faster or slower than the reference clock. One implementation uses two separate counters to count edges of the input clock and edges of the reference clock. Values stored in those counters are then compared after a certain number of reference cycles. Comparison is slow with such an implementation. Certain applications, such as those in a memory interface, do not have a reference clock available. Fast detection compared with a frequency threshold is also needed in certain applications.
It would be desirable to implement a fast-response reference-less frequency detector.